Mariano, Leone
/ Excited-State Chemistry of I -Alkrnyl-2-pyridones
methyl-S-phenylsulfoximinewas obtained as an oil. Butyl phenyl sulfide, bp 125-130 O C : ( 3 mm), was oxidized to the sulfoxide with hydrogen peroxide. The sulfoxide was converted to the oily sulfoximine by hydrazoic acid. The sulfoximine was methylated using formaldehyde and formic acid using the procedure described above. S-Cyclohexyl-N-methyl-S-phenylsulfoximine (8).*O Cyclohexyl phenyl sulfide, bp I 17- 120 'C (I.5 mm), was prepared in 50% yield by the reaction of sodium thiophenoxide wnith bromocyclohexane. Periodate oxidation of the sulfide gave cyclohexyl phenyl sulfoxide. mp 63-65 OC, in 89% yield. Reaction of the sulfoxide with mesitylsulfonyloxyamine gave S-cyclohexyl-S-phenylsulfoximine.nip 74-75 "C, in 65% yield. The sulfoximine was methylated with trirnethyloxonium fluoroborate in the presence of sodium carbonate to give 8 in 80% yield as a viscous oil.
Acknowledgments. This work was supported by a grant from the National Science Foundation. W e thank Dr. James Shanklin for suggestions and assistance with the early stages of this work. Supplementary Material Available: Analytical and spectral data (8 pages). Ordering information is given on any current masthead
page.
3607
References and Notes (1) A. Maercker, Org. React., 14, 270 (1965). (2) B. M. Trost and L. S. Melvin, Jr., "Sulfur Ylides". Academic Press, New York, 1975; E. Block, "Reactions of Organosulfur Compounds", Academic Press, New York, 1978. (3) E . J. Corey and T. Durst, J. Am. Chem. SOC.,88, 5656 (1966). (4) F. Jung, N. K. Sharma, and T. Durst, J . Am. Chem. Soc., 95, 3420 (1973). (5) R. M. Coates and R. L. Sowerby, J. Am. Chem. SOC.,94, 4758 (1972). ( 6 ) T. Shono, Y. Matsumura, S. Kashimura, and H. Kyutoku, Tetrahedron Len., 2807 (1978). (7) T. Mukaiyama and M. Hayashi, Chem. Lett., 15 (1974). (8) M. Julia and J. M. Paris, Tetrahedron Lett., 4833 (1973). (9) M. Julia, personal communication. (10) B. Lythgoe and I, Waterhouse, Tefrahedron Lett., 4223 (1977). (11) A. G. M. Barrett, D. H. R. Barton, R. Bielski, and S. W. McCombie, J. Chem. SOC.,Chem. Commun., 866 (1977). (12) (a)C. R. Johnson, Acc. Chem. Res., 6 , 341 (1973); (b)C. R. Johnson, J. R. Shanklin, and R. A. Kirchhoff, J. Am. Chem. Soc., 95, 6462 (1973). (13) (a) C. W. Schroeck and C. R. Johnson, J. Am. Chem. Soc., 93,5305 (1971): (b) C. R. Johnson, C. W. Schroeck, and J. R . Shanklin, ;bid., 95, 7424 (1973). (14) H. Hoffmann, Arch. Pharm. (Weinheim, Ger.), 306, 567 (1973). (15) A. Schrtesheim, R . J. Muller, and C. A. Rowe, Jr., J . Am. Chem. Soc., 84, 3164 (1962). (16) T. F. Corbin, R. C. Hahn, and H. Shechter, Org. Synth.. 44, 30 (1964). (17) T . Shono, Y. Matsumura. S . Kashimura, and H. Kyutoku, Tetrahedron Lett., 1205 (1978). (18) H. 0.HouseandR. S. Ro, J. Am. Chem. Soc., 80, 182(1958):J. K. Kochi and D. M. Singleton, ibid., 90, 1582 (1968). (19) J. K . Kochi and D. M. Singleton, J. Am. Chem. Soc., 90, 1582 (1968). (20) C. R. Johnson and H. G. Corkins, J . Org. Chem., 43, 4136 (1978).
Excited-State Chemistry of 1-Alkenyl-2-pyridones. Exploratory and Mechanistic Investigations and Kinetic Analysis of Photocyclization Reactions Involving Conversion of the N-Vinylamide to Oxazolium Ylide Function' Patrick S. Mariano*2 and Andrea A. Leone Contribution froin the Depnrtnrent of C'hmisfr,v. Tr.ras A & M L'niwrsitj,. C'ollrgr Stniion. Tr.rc1.s 77843. Krt,ril.cd Aligli.5~13. 1978
Abstract: The excited-state chemistry of I -alkenyl-2-p)ridoner has been subjected to detailed stud), The isobuten) I- ( 1 ) and propenyl- (2) 2-pyridones undergo t h e familiar Dewar-pqridone forming elcctr climtion and ,-I + -4dimerization L+ hen irradiated i n solvents of IOU. polarity. The nature of p a t h u a j s 1'ollol+edb j these t e m s change\ L+ hen rcxtions arc conducted in polar solvents such iis methanol and water. Under these conditions electroc)c ition the .\ -vinqlamidc occurs to generate cventuallq the pqridonylcthanol deriLativcs 7 and 8 froni binglct excited alkenylpqridonch. lrradintions of aqueous solutions of I and 2 containing perchloric acid lead to productiori 01' XLI IO lo [ 3 2-0 1 py r i d i n i u 111 pc rc h I ora t cs ( 9 ;i nd 1 0 ) . I n for ni a t i o n has been accumulated to support mechanisms for thesc pr 'cs involving initial gencr;ltion of azonlethinc qlidcs 28, uhich are trapped by water or h)dronium ion to give oxa7olop!ridiniuin siilts, Thc hqdroxidc salts produccd b) proton transfer from water are then transformed under mild Lvorkup condition> to the corresponding p!ridon!lcth;inols. I>ctniled kinetic analysis of the singlet reaction and deca) pathwaks for the isobuten!l-2-p)ridone has been pcrforiiied using ii varietq o f photochci1iic;il and photophyaicnl techniques. This has yielded individual rate constant\ for singlct-p)ridonc convcrbion to Dcbrar pqridone and pyridinium llidc. singlet decay and emission, and ylidc [rapping b j wciter and return to ground-state pjridone. The interesting solvent polarity dependence o f the nature of photochemical reaction path\\;iy\ follohcd by the I -~rlkcnql-3-p)ridoncs ha!, been explored using measured singlet Iifetiinc~.Iluorc\cence efficiencic\. ,ind rcaction q u a n t u m ields a n d interpreted in terms of a dramatic cfI'cct ol'wlvent polaritq on ratc constants Tor con\cr>ionol'\inglci I -alkcn~l-2-pjridones to the dipolar, azomethine jlides. Solvent polarity effects on singlet lil'ctiinea 01' ;I number of s!stems containing ,\ -vin)l;iniide and related divinylamine chromophores have been noted. Lastly, thc cVfect ofiilk)I bubstitution on I-~iIkenyI-'-p)ridone cqcliration cfl'iciency is discussed in terms o1'ground and singlet excited \i;ite conforinational control. %
Photochemical investigations of systems containing more than one potentially reactive chromophore have uncovered a wealth of information about the physical and chemical characteristics of excited states of organic subst;mces. As a result. organic materials containing numbers of different photochemically reactive groupings continue to scrve ;is appealing 0002-7863/7Y / I ~ O l - 3 6 O 7 $ O 00/0 l
targets for ongoing explorations. An analysis of one such system, the 1 -alkenyl-2-pyridones ( I ) , reveals the potential for diverse types of photochemical behavior d u e to the presence of the p y r i d ~ n e . 1~-vinylaniide,4 oxadi-r-amine,5 and dij ~ - a t i i i n e "chromophores. ~~ Indeed. the excited-state chemistry of systems containing each of these moieties either has been 'c 1979 American Chernicx Socictb
J o u r n a l of the American Chemical Society
3608 Chart I
/
101.13
/
J u n e 20, 1979
m a d e structural assignments simple. Lastly, low-conversion irradiation of the trans-propenylpyridone 2 in these solvents results in cis-trans isomerization.8 However, none of the corresponding Dewar pyridone or [4 41 dimer products from 2 have the cis configuration about the exocyclic propenyl 7r bond. T h e photochemistry of 1 and 2 is markedly altered when irradiations a r e conducted using water or wet methanol as solvent. Accordingly, both 1 a n d 2 a r e transformed in a reasonably efficient (ca. 40-80%) manner to their corresponding 1 -(2-pyridonyl)ethanoI derivatives 7 and 8 when irradiated in water using Pyrex-filtered light. Characterization of the alcohol photoproducts was made by comparison of accumulated physical and spectroscopic d a t a with those for authentic samples previously prepared8 by sodium borohydride reduction of or methyllithium addition to 1 -acetonyl-2-pyridone.
+
o lo I R R well explored or can be speculated about on the basis of analogy. Thus, one might conceive of the pathways outlined in C h a r t I as being potentially available to excited states of 1 alkenyl-2-pyridones. As a result of this interesting feature and the potential synthetic importance of these possible reactions, a n exploration of the excited-state chemistry of 1 -alkenyl-2pyridones was initiated. Our investigations of I -alkenyl-2-pyridones have led quite unexpectedly to the unveiling of a new photochemical reaction involving the N-vinylamide grouping. This report describes the complete details of our studies in this area which have shown that ( I ) I-alkenyl-2-pyridones ( I ) undergo a novel photocyclization reaction to produce oxazolopyridinium ylides ( I I ) , (2) the rate constant for photocyclization of I is strongly dependent on solvent polarity, and (3) trapping of ylides I 1 by acid or water leads to production of cyclization products
(Ill).
m
I Results
Preparative Photochemistry. Irradiations of solutions of I -(2-methyl-l -propenyl)-2-pyridone (l)x and 1 -( 1 -transpropcnyl)-2-pyridone (2)8 in a variety of nonpolar organic solvents including tetrahydrofuran and benzene led to clean production of the isobutenyl and trans-propenyl Dewar pyridones (3 and 4) and crystalline [4 41 dimers ( 5 and 6). T h e
+
RI 3 4
I R , =R2=CH,
2 R, = C H , ,R,=H
+
5
6
Dewar pyridone to [4 41 dimer product ratio, as expected, was found to be dependent upon the concentration of starting alkenyipyridone; maximal yields of ca. 60-80% for 3 and 4 were obtained using concentrations of 1 and 2 of ca. 4 m M while those for the [4 41 dimers were ca. 30% starting with alkenylpyridone concentrations of ca. 30-50 m M . Comparisons of the spectroscopic properties of the alkenyl Dewar pyridones and [4 41 dimers with those recorded for the analogous substances derived from irradiations of 1 -methylpyridone’
+
+
‘A
A
R
hv
I or 2
0
H2O
Hod
7R,=R2=CH3 8R,=CH,,R,=H
RI R2 Interesting changes a r e noted when the courses of the aqueous solution photoreactions of 1 and 2 are monitored using U V spectrophotometric methods. Shifts in the UV maxima of the respective photolysates derived from 1 and 2 were as follows: 309 and 302 nm before irradiation, 288 and 287 nm immediately after irradiation, and 305 and 303 nm after concentration in vacuo. In addition, the long-wavelength shifts in absorption maxima detected when the crude photolysates were concentrated could be induced by the addition of excess potassium hydroxide. These observations suggest that the initial photochemical step of each reaction leads to generation of a short wavelength absorbing intermediate which is converted to alcohol in a secondary dark reaction occurring during the room temperature concentration process and, possibly, involving hydroxide ion. An understanding of these observations was gained when irradiations of 1 and 2 were performed on aqueous solutions containing sixfold molar excesses of perchloric acid. Under these conditions, 45-50% yields of two crystalline salts were obtained by chloroform trituration of the concentrated photolysates. These substances were shown to have physical and chemical properties consistent with their assignment as 5,Sdimethyl- and 5-methyloxazolo[3,2-a]pyridinium perchlorates, 9 ( U V max 285 nm) and 10 ( U V max 284 nm), respectively. Identical materials could be generated by irradiation of 1 and 2 in aqueous formic acid followed by perchlorate ion exchange using Dowex I-X8. Further supportive evidence for the structural assignments of the salts 9 and 10 was gained using chemical methods. Reaction of 9 and 10 in methanolic potassium hydroxide a t room temperature led to smooth production of the pyridonylethanol derivatives 7 and 8. Thermolyses of the pyridinium perchlorate
M a r i a n o , Leone
/
Excited-State Chemistry of I -Alkenyl-2-pyridones
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salts a t > 150 OC led to rapid generation of the known8 methallyl- and allylpyridones 11 and 12. Lastly, both 9 and 10 were efficiently converted to the 1 -alkenyl-2-pyridones, 1 and 2 (mixture of cis and trans isomers), when stirred with potassium tert-butoxide in tert-butyl alcohol. Dark control reactions of the alkenylpyridones 1 and 2 were attempted using conditions which mimic those present during the photochemical reactions in water or aqueous acid. I n no cases were the pyridonylethanol derivatives or pyridinium perchlorates detected and unreacted starting materials were recovered quantitatively. In addition, irradiations of 1-vinyl2-pyridone (13),1°using conditions found to be ideal for photoconversion of 1 a n d 2, did not produce isolable quantities of I -(2-pyridonyl)ethanoI (14) or the oxazolo[3,2-a]pyridinium salt 15. ao
0.1
az
a3
OA
0.5
[WENCHER] (M)
13
15
14
Mechanistic Aspects. Triplet photosensitized reactions of isobutenylpyridone 1 were attempted using xanthone ( E T = 74.2 kcal/mol)l l a and Michler’s ketone ( E T = 6 0 kcal/mol)I l a as sensitizers in wet methanol a s solvent and employing conditions which guaranteed light absorption by the sensitizer and high triplet energy transfer efficiencies. In each case, unreacted starting material was quantitatively recovered. It is certain that energy transfer from triplet xanthone to 1 was occurring, since results from parallel irradiations of methanolic solutions of xanthone and 1, and xanthone alone, indicated the ability of 1 to quench the familiar xanthone triplet photoreduction reaction.’ I Additional information necessary to assign the singlet as the reactive 1-alkenyl-2-pyridone excited state undergoing photocyclization and alcohol production was gained from quenching studies using 2,5-dimethyl-2,4-hexadiene, a known singlet quencher.12aPhotoreactions of 1 in methanolic acetonitrile solutions containing this diene in concentrations ranging from 0 to 0.5 M were conducted under simultaneous irradiation conditions. A plot of the ratio of unquenched t o quenched relative quantum yields for alcohol 3 production vs. diene concentration gave a straight line with slope (1.9 M-’ with u = 0.89) approximately equal to those obtained from plots of corresponding unquenched to quenched fluorescence q u a n t u m yield (2.1 M-’ with u = 0.95) and singlet lifetime ratios (2.1 M-I with u = 0.99) (see Figure I n a n attempt to further explore the mechanistic details of the novel photochemical reactions observed, the I-alkenyl2-hydroxypyridinium perchlorates, 16 and 17, were indepen16 R,=Rz=CH3 17 R, =CH,,R,=H
dently prepared from the corresponding pyridones by treatment with perchloric acid. Irradiation of aqueous methanolic solutions of 16 and 17, containing 0.54 M perchloric acid to ensure >99% of the pyridinium salt form, led to complete consumption of these substances. However, none of the corresponding oxazolo[3,2-a]pyridinium salts was formed in either case. Likewise, the methallyl- and allylpyridones, 11 and 12, remained unreacted when irradiated in water for time periods which would have led to complete reaction of 1 and 2. On the basis of the results presented thus far, it is possible
Figure 1. Quenching of the fluorescence lifetime and photohydration reaction of I -(2-methyl-I -propenyl)-2-pyridone by 2,S-dimethyl-2,4-hexadiene. Plotted are the ratios of unquenched to quenched singlet lifetimes (0).fluorescence quantum yields (O), and reaction quantum yields ( A ) vs. quencher concentration.
to conclude that the singlet transformations to pyridonyl alcohols and oxazolopyridinium salts require that the starting pyridone contain an N-vinylamide grouping. A n important exception to this is the parent I-vinylpyridone which is lacking alkyl substitution a t the terminal carbon of the exocyclic vinyl grouping. I n order to clarify the apparent critical role played by alkyl substituents, information about the relative efficiencies for reaction of the cis- and trans-propenylpyridones was sought. This turned out to be quite difficult owing to the competitive cis-trans isomerization process which efficiently interconverts both isomers. However, qualitative evidence obtained from varying conversion and competitive irradiation experiments seems to indicate that transformation of the cis isomer to alcohol 8 is slightly more efficient than that of the trans isomer. Lastly, the divergent photochemical behavior of the I-alkenyl-2-pyridones in nonpolar organic solvents a s compared to methanol and water required closer inspection. Dewar pyridone 3 and alcohol 7 production from fixed-time irradiations of 1 were monitored a s a function of the amounts of water in the tetrahydrofuran solutions used. T h e data obtained from these experiments, plotted in Figure 2, clearly shows that the efficiency of alcohol production increases a t the expense of Dewar pyridone as the percentage of water in the aqueous tetrahydrofuran solvent is increased. I n addition, the quantum efficiency for production of Dewar pyridone 3 from isobutenylpyridone 1 was found to decrease dramatically as solvent polarity increases (Table I).
Quantitative Photochemical and Photophysical Data. Q u a n t u m efficiencies ( @ r ) for alcohol 7 and pyridinium salt 9 production from isobutenylpyridone 1 were measured using a “linear optical bench” apparatus and potassium ferrioxalate actinometry.I3 Necessary precautions were taken to prevent singlet self-quenching and secondary reactions of the products (conversions of 8- 18%), to ensure that all light is captured by actinometer or photoreaction substrate, and to guarantee that the filter solutions used to isolate desired wavelength regions remained transparent throughout irradiations. Product analyses for reactions conducted in aqueous acetonitrile solutions were performed on the crude photolysate after concentration in vacuo, while those performed i n aqueous perchloric acid reactions required prior treatment of the crude photolysate with stoichiometric amounts of potassium hydroxide. This latter procedure was employed to convert quantitatively the
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ID0
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5.00
[H30+l (mM)
in THF (''/VI
01. HO,
Figure 2. The relative efficiencies for lormation of isobutenyl Dewar pyridone (m)and I -pyridonyl-2-methyI-2-propanol (A)and disappearance o l isobutenyl-2-pyridonc a s a function of water concentration in aqueous tctrahydrofuran solutions.
Table I . Quantum Yields and Rate Constants ( 2 5 "C) for D w a r Pyridone Formation from I-Isobutenyl-2-pyridone in Solvents of V 'i r y i ng Polarity
solvent ET valueb
CHiOH CHiCU TH F
37.4 46.0 55.5
-
( b ~ p
solvent"
1
3
0.061 0. I43 0.260
7,
k
~
nsL
(25 p "C). S-'
7.3 f 1.0 16.6
8.36 X IOh 8.61 X IOh 8.67 X IO6
30.0
Solvents were rendered anhydrous by rigorous drying and distillation prior to use. ET values are those given by K. Dimroth et at.. Jirstus Liebigs Ann. Chew., 661, 1 (1962). Degassed, nitrogenpurged solutions were used a t 25 OC.
primary photoproduct, oxazolopyridinium perchlorate 9, to the more easily analyzed alcohol 7. T h e d a t a obtained from irradiations of 1 in solvents containing varying water and perchloric acid concentrations (Table 11) show that 4,. increases dramatically as either water or acid concentrations increase (see also Figure 3). In order to examine the source of these changes, fluorescence q u a n t u m yields ($r) and singlet lifetimes ( T ) for 1 were measured under conditions identical with those employed for the 6,. determinations. 4,-and 7 were found to be dependent upon [ H 2 0 ] but to be invariant with changes in [ H 3 0 + ] (Table 11). I n addition, through the use of this data it is possible to calculate the rate constant for fluorescence of l ( k f = @ f / 7 ) . T h e changes noted in 4,- a s a function of [ H 2 0 ] appeared interesting and worthy of further exploration. Accordingly, the fluorescence efficiencies of 1, in a variety of single-component solvents (Table 111), and ethanol-water mixtures of differing polarity (Table IV), were recorded. This data reveals a clear inverse dependence of & o n solvent polarity. I n order to test the generality of this phenomenon and to probe for its source, singlet lifetimes of a number of 1 -alkenyl-2-pyridones (2-cis, 2-trans, 13, MI4) and related methallylpyridone (1 l ) , along with a variety of substances containing the isoelectronic N-vinylamide (19 and 20)" and di-n-amine (21 and 221h)
19 R = CH,
21 R = P h
20R=Ph
22 R = I cyclo hexsnyl
-
I;igure 3. Q u a n t u m cfficiency for production o f 5.S-dimeth1 loxa7olo[3,2-a]p~ridiniumperchlorate as a function o f hydronium ion concentration from irradiation of 1 -(2-meth)l- I -propcnql)-2-p!ridone
i n aqueous perchloric acid solutions.
chromophores, were measured in water, 1 :1 water-acetonitrile, and acetonitrile solutions (Table V).
Discussion Mechanistic Features. Several possible mechanisms can be envisaged to rationalize the photochemical reactivity of 1 alkenyl-2-pyridones in polar protic solvents. It appears almost certain that the initial photoproducts from irradiations of 1 and 2 under these conditions a r e oxazolo[3,2-a]pyridiniumsalts. W h e n produced in acid solution, these salts (9 and 10) possessing the nonnucleophilic perchlorate counterion are sufficiently stable to survive isolation and purification. The primary photoproducts produced from reactions of 1 and 2 in water have U V spectroscopic characteristics indicative of cyclized pyridinium salt structures 23 and most probably contain hydroxide as the counterion ( Z = O H ) . T h e secondary dark reactions leading to the pyridonylethanol products, occurring upon concentration of or by addition of hydroxide to solution containing the pyridinium salts 23, most probably involve nucleophilic attack a t the pyridinium C-2 position followed by uncoupling of the hemiketal linkage in the transient dihydropyridine 24.
23
24
T h e more important mechanistic question, however, concerns the nature of the pathway(s) used by singlet l-alkenyl2-pyridones for transformation to oxazolopyridinium salts. Simple photohydration" of the exocyclic vinyl groupings in 1 and 2 followed by dehydrative cyclization appears an unlikely possibility since the pyridonyl alcohols 7 and 8 show no tendency to cyclize to the pyridinium salts in aqueous or dilute acid solutions used for the photoreactions. Another mechanism, worthy of more serious consideration, finds analogy in the results of studies by SchmidI* and Horspool1yof the photocyclization reactions of o-allylphenols to benzodihydrofurans. These transformations are most probably d u e to the enhanced facilities for intramolecular proton transfer i n the excited-state manifold of the allylphenols. Accordingly, oxazolopyridinium salts may arise from analogous pathways involving intramolecular proton transfer in initially formed singlet excited hydroxypyridinium salts followed by
Mariano, Leone / Excited-State Chemistry of I -Alkenyl-2-pyridones
361 1
Table 11. Experimental and Calculated Photophysical and Photochemical Properties of I -1sobutenyl-2-pyridone
water concn,' M 27.7 50.0 55.6 55.6 55.6 55.6 55.6 55.6
HC104 concn, mM
singlet lifetimes, 7, ns
fluorescence quantum yields,c @ f
Reaction quantum yields,d Gr
fluorescence rate constants kf, s-'
0 0 0 0.50
10.3 f 1.0 6.5 f 1.0 5.1 f 1.0
0.007 f 0.001 0.004 0.003
0.07 1 0.1 I O
7 x 105 6 X IO5 6 X IO5
0.130 0.160 0.190 0.213 0.248 0.3 I 5
1 .oo
2.00 3 .OO
5.00
0.003
5.1 f 1.0
6 X IO5
f' Acetonitrile was used as cosolvent for solutions containing less than 55.6 M water. Singlet lifetimes were measured using degassed, nitrogen-purged solutions at room temperature with a single photon counting nanosecond spectrofluorimeter based on ORTEC components. Values for 7 were calculated by a digital RX-01 computer using the method of moments program. Fluorescence quantum yields were measured using degassed, nitrogen-purged solutions at room temperature with a Spex-Fluorolog in the E/R mode. Benzene and naphthalene were used as fluorescence standards for @f determinations. d Reaction quantum yields were measured for product production using degassed, nitrogen-purged solutions and an optical bench apparatus, Light output was determined using potassium ferrioxalate actinometry. Alcohol product analysis w a s performed by GLC using internal standards. The pyridinium salts obtained from irradiation of acid solutions were first converted to alcohol by treatment with base before analysis. Conversions to product were in the range of 1 1 - 1 7%. Estimated error for br measurements is 5%. Irradiations of aqueous solutions of the isobutenylpyridone in the absence of acid lead directly to the pyridonyl alcohol. Table 111. Fluorescence Quantum Yields for I-lsobutenyl-2-
Table IV. Fluorescence Quantum Yields for I-Isobutenyl-2-
pyridone in Single Component Solvents of Varying Polarity
pyridone in Aqueous Ethanol Solutions
fluorescence quantum yields, @ f '
solvent
solvent Z valueb
0.003 0.017 0.024 0.03 I
H2 0 EtOH DMF Me7SO
94.6 79.6 68.5 71.1
Solutions used for these measurements were degassed and nitrogen purged. Benzene and naphthalene were used as fluorescence standards. Estimated errors are f O . O O 1 . 2 values for solvents are those reported by E. M. Kosower, J . Am. Chem. Soc.. 80, 3253 (1958). rapid cyclization of the intermediate pyridonylethyl cations 25. Although this mechanism appears attractive since it nicely
16 or 17
25
serves to rationalize the large differences in reactivity between the isobutenyl and vinylpyridone systems, several critical observations appear to limit its serious consideration. T h e concentrations of 16 and 17 in water or dilute acid solution should be exceptionally low owing to the expected large pKb of Nsubstituted pyridones (pK,s of 1-methyl-2-hydroxypyridinium salts are 0.3).'" Ultraviolet measurements on aqueous solutions of 1 and 2, and 16 and 17, strongly support this expectation and further demonstrate that the a m o u n t of light competitively absorbed by the shorter wavelength absorbing pyridinium salts under the reaction conditions would be exceedingly low. Likewise, proton transfer from water to singlet excited I-alkenyl-2-pyridones as a mechanism for formation of 16 and 17 can be deemed improbable on the basis of the results of Bridges and co-workers,2' which show that the singlet state pK,s of 0-protonated 2-pyridones a r e in the range of -4, and our observations, which demonstrate that fluorescence of 1 is not quenched by acid. Lastly, the hydroxypyridinium perchlorates 16 and 17 did not serve as photochemical precursors of the oxazolopyridinium salts. Likewise, the methallyl- a n d allyl2-pyridones, 11 and 12, which a r e closer analogues of the allylphenol systems studied earlier,'8~'9 are also unreactive when irradiated i n water or aqueous acid solutions.
fluorescence quantum yields, @ f a
ethanol content, % v/v
0.017 0.0 I6 0.015 0.014 0.013 0.012
100
solvent 'k valueb -2.033 -0.747
90 80 70 60 50 40 30 20
0.01 1
0.009 0.007 0.005 0.003
0.000
0.595 1.124 1.660 2.196 2.720 3.05 I 3.310 3.493
IO 0
Solutions used for these measurements were degassed and nitrogen purged. Benzene and naphthalene were used as fluorescence htandards. Estimated errors are f O . O O 1 . Y values are those derived by Grunwald and W i n ~ t e i nusing ~ ~ measured rate constants for solvolysis on tert-butyl chloride in aqueous ethanol solutions.
I n considering other likely candidates for the mechanism operating in converting singlet I -alkenyl-2-pyridones to oxazolopyridinium salts, it is instructive to consider the structural and electronic makeup of the center in these substances a t which reaction is occurring. T h e N-vinylamide moiety 26,
s
.RD 9,
R'Y 0N j ) hv 26 27 comprising the exocyclic alkenyl and endocyclic amide groupings, is isoelectronic with chromophores found in divinyland diarylamines, ethers, and sulfides all of which possess patterned excited-state r e a c t i ~ i t y . ~ . ~Thus, ~ . ' ~ the related N-vinylamide function, when present i n appropriate environments established by structural features and solvent, could undergo electrocyclic closure to produce oxygen-substituted, cyclic azomethine ylides 27.24 Rased upon these thoughts, we suggest that the conversion of 1 -alkenyl-2-pyridones to oxazolo[3,2-a]pyridinium salts can best be explained by a mechanism involving initial singlet excited state electrocyclization to generate the pyridinium ylides 28. T h e ylide intermediates could then be rapidly trapped by
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Table V. FYuorescent Quantum Yields and Lifetimes of a Series of I-Vinyl-2-pyridones and Analogous Compounds Containing the Enamide and Related Chromophores in Solvent Systems of Differing Polarity emission monitored, nm (all solvents)
compd
395 395 395 395 395 395 395 400
1
2-cis 2-trans
13 11 19 20 18 21 22 IY,,~.dimeth~laniline
fluorescence efficiencies" emission max nrn, $f
singlet lifetimes, n s a 7
T
CH3CN
50% CH3CN-H20
16.5
10.3 5.7
14.1
15.7 26.2 11.2 18.1
T
H2
6.1
15.0
300
20.1
300 300
27.7 28.8
15.1 15.7
CH3CN (HzO)
tH2O)
395 (387) 400 (395) 403 (395) 405 (395) 380 (370) 408 (390) 405 (395)
0.003 0.057 0.130 0. I47 0.01 2 0.005
5.1 2.5 3.4
24.3 11.8 13.1
16.5 5.9
0
25.1
13.7 12.7 11.9
0.025
0.4
13.5 17.8 26.3
29.9
f1 Solutions used for these measurements were degassed and nitrogen purged. Estimated error is 51.0 ns. and nitrogen purged. Benzene and naphthalene were used as standards. Estimated error is f O . O O 1 .
Solution used were degassed
proton transfer from water or hydronium ion, or in competition could revert to starting pyridone by the retro process. Inclusion of the latter pathway for ylide disappearance is dictated by our observations on the reactions of 9 and 10 with potassium zut-butoxide, which most probably proceed via 28. Additional
kret
support for the ring-opening process comes from studies by Huisgen and his c o - w o r k e r ~that ~ ~ demonstrate ~ a similar fate for monocyclic ylides of general structure 27. Although not explored experimentally, ylide formation and return to ground-state pyridone might well be the major if not exclusive pathway for singlet cis-trans isomerization of 1 -alkenyl-2pyridones, or, more generally, of N-vinylamides.4 Another important feature of this process, especially in regard to the validity of methods chosen for kinetic analysis (vide infra). concerns the manner in which proton transfer occurs to the carbanionic center in 28. Howe and R a t t have ~ ~ ~suggested on the basis of deuterium exchange results that the pK, of ring CY protons of N-alkylpyridiniurn salts is lower than that of the A'-alkyl LY protons. If this is the case, and intramolecular proton transfer converts 28 to the related ylide 29 in competition with return to pyridone, the product-formation step from 28 would have a rate independent of the nature and concentration of the external proton source. However, irradiation of 1 conducted in D?O results in the exclusive production of the oxazolo-ring monodeuterated alcohol 30a, thus ruling out a n internal proton switch mechanism for salt production. I f the latter pathway were functioning, pyridone ring deuterated alcohol 30b would have been generated.
0:-a
%
Z Rl
RQD
RO&H
R2
29
o m
R,
30a
R, Rl 30b
Kinetic Analysis. As is suggested by the mechanism postulated above, the quantum efficiency for conversion of 1 -alkenyl-2-pyridones to oxazolo[3,2-a] pyridinium salts should be dependent on the nature and concentration of proton donors i n solution. Partitioning of the ylide to pyridinium salt rather
28
+
H30+$9
than to ground-state pyridone should be enhanced by inclusion of higher concentrations of acid in the reaction medium since proton-transfer rate constants ( k p ) from H 3 0 + t o 28 a r e expected to be close to diffusion controlled (kd,ff(H@) = 1 X 1 O l o M-l s - l ) . l h In contrast, the bimolecular rate constant for protonation of 28 by H20 (kpo)should lie in the range of 1 X IO6 M-I s - I a t room temperaturezh d u e to an expected p K , of 9 and 10 of ca. Indeed, as inspection of Figure 3 indicates, the quantum efficiency for production of 9 from 1 (&) rapidly increases as perchloric acid concentration in the range of 0 to 5 m M is increased. T h e dependence of quantum efficiency on [ H 3 0 + ] is expressed mathematically in the relationship derived for @, (eq 1 ) using the kinetic sequence shown i n C h a r t 11, steady-state assumptions for Is!and 28, and the definition of 7 = l / ( k d kDp k f + k c ) . It is instructive to point out here that both the singlet lifetime ( T ) and fluorescence quantum yield (&) of 1 are independent of [ H 3 0 + ] , suggesting that alternate explanations for the dependence of $r on acid concentration involving protonation of 1'1 cannot be invoked. This kinetic treatment provides us with a rare opportunity to dissect out
+
+
--&[
_1 4r
1
7kc
1
Tkc kp,
]
[Hz+ ~] kp[H30+]
(1)
the individual rate constants for 151 cyclization ( k c ) ,ylide 28 return to 1 (k,,,), and protonation of 28 by H20 (k,) and thus to obtain nearly all of the rate constants for processes involved
Mariano, Leone / Excited-State Chemistry of I -Alkenyl-2-pyridones
361 3
Table VI. Summary of t h e Rate Constants ( 2 5 OC, HzO) of Processes Used in Deactivation and Reaction of lsobutenylpyridone (1)
Derived from ( k d 4-k ~ p value ) found using solvent-associated linear free energy treatment of &data and value of k ~ p . Assumed for proton transfer from H 3 0 + to 28.26 0.00 050
LOO
I50
200 250
390
350
Y
in deactivation and reaction of lS1.Accordingly, nonlinear least-squares analysis of the Gr vs. [H30+] data provided in Table I gives an excellent fit to eq 1 when the values for k,, kret, a n d k,, a r e those shown in Table VI. Several important aspects of this kinetic analysis require comment. T h e nearness of the value found for k,, to that predicted using the Eigen formulation,26 interrelating the rate constant for proton transfer from H z 0 to 28 to the bimolecular diffusion rate constant, K , , and K , of the conjugate acid, suggests that the statistical method employed to obtain the individual rate constants is reliable. T h e comparison between the value derived for k c , although having associated with it a reasonably large error, and l / r = (kd k ~ p k f k,) demonstrates that the major pathway for 1'' decay in aqueous solution involves cyclization to the dipolar intermediate, 28. Thus, the postulate made above that a significant contribution to the mechanism for cis-trans isomerization of 2 is through the ylide 28 is convincing. One of the more surprising features of these results is the exceptionally large rate constant found for return of the photochemically generated oxazolopyridinium ylide to starting alkenylpyridone. A comparison between the rate constant of 4 X IO8 s - I for the present example and ca. 10'- 1 O6 s-I suggested for the analogous reaction transforming the related dihydrocarbazole ylide 31 to diphenylmethylam-
+
+ +
C,", H
H 31
ine2x serves to further emphasize this intriguing aspect of 1alkenyl-2-pyridone photocyclizations. Solvent Polarity Effects. One of the more significant consequences of observations made during the course of our investigations relates to the dramatic role played by solvent in controlling the photochemical reaction pathways followed by 1 -alkenyl-2-pyridones. W e have shown that direct irradiation of 1 and 2 in solvent systems of low polarity, such an C H 3 C N and C6H6, leads exclusively to formation of Dewar pyridone a n d [4 41-dimeric products, i n a concentration-dependent ratio, and that pyridonylethanol derivatives result from reactions conducted i n wet methanol and water. Moreover, the efficiency of isobutenyl Dewar pyridone 3 production is reduced in a continuous fashion as [HzO] in H'O-THF solutions is increased (Figure 2). Simultaneously, the efficiencies of disappearance of 1 and production of 7 are increased as [ H 2 0 ] is increased. Moreover, the quantum yield for Dewar pyridone production from 1 decreases dramatically as solvent is varied from the less polar T H F to the more polar C H 3 C N and C H 3 0 H . It is quite likely that these observations are reflective of an interesting effect of solvent polarity on partitioning of lsl
+
Figure 4. Fluorescence q u a n t u m efficiencies for I B P in ethanol-water
solutions plotted in the logarithmic form of eq 3 as a function of the solvent Y values using c = 63.9.
to ylide 28 and Dewar pyridone 3, Le., on the kDp/kc ratio. This effect is almost certainly d u e to changes in the rate constant ( k , ) for formation of the dipolar ylide caused by variation in medium polarity. Hammond and Sharpzy have shown that the quantum yield for Dewar pyridone production from 1methyl-2-pyridone, and thus the rate constant, is solvent polarity independent. Likewise, we have shown that kDp for the isobutenyl-2-pyridone 1 is solvent polarity independent and, thus, not responsible for the changes noted. T h e solvent-polarity dependence of k , should be reflected in solvent effects on the singlet lifetimes (7 = l / ( k d 4- kDp k f k,)) and fluorescence efficiencies (& = k f T ) of 1, as shown in Tables 11-V. A reasonably simple, quantitative treatment of the solvent polarity effects has turned out to be quite informative. The solvent associated linear free energy of activation relationship, suggested by Grunwald and Winstein,30applied to the ylide forming process is shown in eq 2,
+
+
where k , and k,, are the cyclization rate constants in aqueous ethanol and 20% H 2 0 - E t O H (v/v), m is the reaction parameter reflecting the sensitivity of k , to changes in solvent polarity or the degree of charge separation in the transition state for lbl 28, and Y is the solvent polarity parameters. Substitution for k, and kcO by functions containing the easily measured &gives eq 3 in which c = 1 ((kd kDp)/kf). Analysis of the data for 4,vs. Y shown in Table IV, using nonlinear least-squares methods. in which the best ni and c values are found, gives an exceptionally close fit to eq 3 when
-
+
1
log - - c = mY (if
+ log
-- c
+
1+
(3)
the reaction parameter m is 0.49 f 0.07 and ( k d kc,p)/kr is 6 3 f 1 . T h e results of this analysis are displayed i n Figure
4. T h e large value of m obtained for cyclization of 1'1 when compared to those observed for ground-state solvolysis reactions (ca. 1) strongly suggests that a significant polarity change is occurring in proceeding from the singlet pyridone to the transition state for ylide formation. It seems reasonable to postulate that this is reflective of a late transition state for conversion of 1'1 to the ylide 28 in either its ground or singlet excited state. The possibility that the large m value stems not necessarily from a transition state with a large degree of charge separation put, rather, from a reversed polarity in 1'1 appears unlikely since the fluorescence maximum and band shape of 1 a r e unchanged over a large range of solvent polarities.
3614
J o u r n a l of the American Chemical Society
In summary, the solvent polarity treatment described above offers additional support for the proposed mechanism for conversion of 1 -alkenyl-2-pyridones to oxazolopyridinium salts. It is clear from the fact that k , varies ca. four orders of magnitude in changing solvent from T H F 3 ' to H20 a n d ca. two orders of magnitude from E t O H to H 2 0 that this process would be an insignificant competitor of Dewar pyridone formation in photoreactions of alkenylpyridones conducted in nonpolar organic solvents. Lastly, from the value of (kd k D p ) / k f obtained from this treatment it is possible to calculate the rate constant for radiationless decay of lSI( k d ) (see Table
+
VI). Structural Effects. Another interesting feature of the results presented above concerns the apparent substituent effects on the reactivity of singlet 1-alkenyl-2-pyridones. O u r preliminary results indicate that the efficiency for oxazolopyridinium salt formation is critically dependent on both the number and geometrical location of substituents on the exocyclic vinyl moiety, Le., a qualitatively judged order of reactivity of isobutenyl > cis-propenyl > trans-propenyl >> vinyl. Importantly, the source of the reactivity differences might lie in substituent effects on the rates of ylide formation from the singlet pyridones, a s judged by the correlation between reactivity and solvent polarity effects on singlet lifetimes and fluorescence efficiencies of the 1-alkenyl-2-pyridones. Although a rationalization for the substituent effects on k , is not readily apparent, information which might be suggestive of the source of the control is found in earlier studiess on the preferred C - N conformations of 1-alkenyl-2-pyridones. Spectroscopic d a t a presented a t that time showed that the presence of cis-alkyl substitution of the terminal vinyl carbon of I-vinyl-2-pyridones causes bisected conformers, 32, to be preferred. In confor-
32
33
mations of this type, the terminal vinyl carbon is preparatorily oriented in the direction of motion required for cyclization. Moreover, the preferred C - N conformations in cases lacking large cis-alkyl substituents, such a s the parent vinylpyridone 13, are known8 to be planar and to most probably have the vinyl moiety oriented anti with respect to the amide carbonyl. Conformations of this type, 33, for singlet excited systems would of course be incapable of cyclization. Thus, the substituent effects may be stereoelectronic or conformational in nature and may be manifested in terms of least motion or orbital overlap control. Further studies should clarify these questions. Lastly, if our interpretation of the solvent effects is correct, then changes in singlet lifetimes and fluorescence q u a n t u m yields of other substances containing the N-vinylarnide or related diaryl- or divinylamine chromophores might serve as good indicators of when singlet states of these systems have available to them reaction pathways involving electrocyclization to form dipolar ylides. Processes of this type have been invoked to rationalize the triplet photochemistry of the enamine 22' and diphenylmethylamine.28This feature has been preliminarily investigated using the acyclic enamides, 19 and 20, and amines 21 and 22 (Table V ) . For all cases in which theN-vinylamide and diaryl- or arylvinylamine chromophores are present, small but definite decreases in the singlet lifetimes were noted when the solvent system was made more polar by changing from acetonitrile to water.'?
Summary T h e results of studies conducted thus far in the area of 1 -
/
101:13
1 J u n e 20, 1979
alkenyl-2-pyridone photochemistry have led to the unmasking of a novel photocyclization reaction involving the N-vinylamide chromophore. Although only tentative, the results indicate that this excited-state process may well be quite general and operable in a number of systems containing this and related chrom o p h o r e ~ . ~I n~ addition -~~ the study has demonstrated the wealth of information that can be made available from detailed analyses of photophysical and photochemical data obtained by employing the modern tools of photochemistry. Moreover, the 1 -alkenyl-2-pyridones should serve a s useful models in future studies aimed a t probing the detailed electronic and structural features of excited-state processes.
Experimental Section Elemental analyses were performed by Galbraith Laboratories, Knoxville, Tenn. NMR spectra were recorded on Varian HA-IO0 or T-60 (proton) and JEOL PS-100 (carbon) spectrometers with tetramethylsilane as an internal standard. Infrared spectra were taken using Beckman IR-8 or Perkin-Elmer 237 spectrometers using polystyrene as reference. Ultraviolet absorption spectra were measured using a Beckman Acta-Ill spectrophotometer. Gas chromatographic analyses were performed with a Varian Model 940 chromatograph with flame ionization detection. Mass spectroscopic measurements were made at 70 eV on a CEC-21- I I O double-focusing, high-resolution mass spectrometer. Melting points were obtained uncorrected using a Griffin Mel-Temp apparatus. Preparative irradiations were conducted in an apparatus consisting of a Hanovia 450-W mediumpressure lamp surrounded by the appropriate glass filter within a quartz, water-cooled immersion well surrounded by the solution being irradiated. Photolyses requiring simultaneous irradiation of multiple samples were performed using the lamp housing, described above. located at the center of a merry-go-round apparatus containing the tubes with samples. Preparative TLC was performed using 20 X 20 cin plates coated with silica gel (E. Merck, GF-254, type 60). Fluorescence measurements were made using a Spex-Fluorolog emission spectrometer. Irradiation of 1-(2-Methyl-I-propenyl)-2-pyridone(1) in Nonaqueous Solvents. Preparation of the lsobutenyl Dewar Pyridone 3. A solution of 1 -(2-methyl- I-propenyl)-2-pyridone8 (250 mg, I .68 mmol) in 450 mL of degassed tetrahydrofuran was irradiated in a preparative apparatus for 2.0 h using Pyrex glass filtered light. Nitrogen was bubbled through the solution during the irradiation. The crude photolysate was concentrated in vacuo, giving a golden oil which was subjected to preparative TLC (50% ethyl acetate-chloroform). This purification method gave I50 mg (60%) of pure 2-(2-methyl-l-propenyl)-2azabicyclo[2.2.0]hex-5-en-3-oneas a golden oil: IH NMR (CDCI,) 6 6.64 (m. 2 H, H-5 and H-6), 6.02 (m. 1 H, isobutenyl), 4.72 (m, I H, H-l),4.19 (m. 1 H, H-4), 1.85 (s, 3 H. CH3), and 1.75 (s, 3 H, CH3): I3C NMR (CDC13) 6 167.0(s,C-3), 141.8and 140.7 (d,C-5 and C-6). 120.5 (s, isobutenyl quaternary). 117.9 (d, isobutenyl methine), 57.6 and 56.0 (d, C-l and C-4). 23.0 and 17.4 (q, methyls): UV (95% EtOH) max 228 nm ( E I2 800); IR (liquid film) 3000, 1725. 1635. 1400. and 1370 cm-I; mass spectrum m / e (re1 intensity) 149 (P, 9). 134 (100). 120 (19). 107 (26). and 94 (17); high-resolution mass spectrum I H / C J 149.084 06 (calcd for CsH1 {NO, 149.084 75). Preparation of the Bisisobutenyl[4 41 Dimer 5. A solution of I S O g (10.1 mmol) of I-(2-methyl-l-propenyl)-2-pyridone8in 200 mL of degassed tetrahydrofuran was irradiated in a preparative apparatus using Pyrex glass filtered light for 2.0 h . Nitrogen was bubbled through the solution during irradiation. Concentration of the crude photolysate in vacuo to ca. 20 niL followed by the addition of ca. 20 mL of ether caused precipitation of the [4 + 41 dimer 5 as a white, crystalline solid (0.96 g, 64%): mp 227-228 "C; ' H N M R (CDCI,) 6 6.80 (m. 2 H, endocyclicvinyls), 6.30 (t, 2 H. endocyclic vinyls), 5.7 (d, 2 H, isobutenyl), 4.2 and 3.6 (m, 4 H, bridgehead), 1.8 and I .6 (s. 12 H, methyl); mass spectrum n7/e (re1 intensity) 149 (P, 69), 148 (44). 134 (100); high-resolution mass spectrum base peak m/e 149.084 46 (calcd for C ~ H I I N O149.084 , 06). Irradiation of 1-( l-trans-Propenyl)-2-pyridone ( 2 )in Nonaqueous Solvents. Preparation of the trans-Propenyl Dewar Pyridone (4). Irradiation through Pyrex glass of I -( I-trans-propenyl)-2-pyridone8 (500 mg. 3.70 mmol, in 850 mL of tetrahydrofuran), using the conditions, workup, and purification procedure described above for preparation of the Dewar pyridone 4, gave 414 mg (83%) of pure 2-
+
Mariano, Leone / Excited-State Chemistry of I -Alkenyl-2-pyridones ( I -trans-propenyl)-2-azabicyclo[2.2.0]hex-5-en-3-one as
a clear oil: ' H NMR (CDC13) 6 6.70 (m, 2 H, H-5 and H-6), 6.25 (d. 1 H, propenyl H-I), 4.85 (m, 2 H, H-1 and propenyl H-2), 4.25 (m. I H, H-4), 1.9 (d. 3 H, CH3); I3C N M R (CDC13) 6 166.7 (s, C-3), 142.2 and 140.4 (d. C-5 and C-6), 122.0 (d, propenyl C-I). 107.1 (d, propenyl C-2), 58.3 and 56.2 (d. C-l and C-4), 1 I .6 (q, CH3); UV (95% EtOH) max 242 nm ( e 1 1 400): IR (liquid film) 2900,1725,1665,1535, and I360 cm-I. Attempts at obtaining elemental analytical data for this material failed owing to exceptional difficulties with purification and the low intensity of the parent ion in the mass spectrum. Preparation of the Bispropenyl [4 + 41 Dimer 6. Irradiation of a degassed, nitrogen-purged solution "cf 1 -( 1 -trans-propenyl)-2-pyridone8 (3.00 g, 0.022 mmol) in 800 mL of acetonitrile in the preparative apparatus for 0.5 h gave after concentration in vacuo and addition of ether I .OO g (33%) o f the [4 + 41 dimer 6 as a white, crystalline solid: mp 216-217 "C: IH NMR (CDC13) 6 6.90 (m, 2 H, propenyl H-I). 6.6 (m, 4 H, endocyclic vinyls), 5.3 (m,2 H, propenyl H-2), 4.5 and 3.7 (m, 4 H. bridgehead), I .7 (d, 6 H, CH3); high-resolution mass spectrum base peak wile 135.068 86 (calcd for CgHqNO, 135.068 4 I ). Irradiation of .4queous Solutions of 1-(2-.Methyl-l-propenyI)-2pgridone. Preparation of I-(2-Hydroxy-2-methyIpropyl)-2-pyridone (7). A solution of 1 -(2-rnethyl- 1 -propenyl)-2-pyridone8 ( 1 .OO g, 6.7 mmol) i n 2.0 L of degassed. deionized water was irradiated in the preparative apparatus for 2.0 h using Pyrex glass filtered light. Nitrogen was bubbled through the solution during irradiation. Concentration o f the photolysate in vacuo gave 0.98 g of a red oil ( 8 8 % ) characterized by spectroscopic methods as I -(2-hydroxy-2-methylpropyl)-2-pyridone.8 U V absorption spectrophotometric monitoring of the photolysis and workup showed the following changes: max 309 (before irradiation), 288 (crude photolysate before concentration), and 305 nm (after concentration). The colorless. crystalline pyridonyl alcohol 7 (0.61 5 g. 55% mp I 19- I 19.5 "C). identical in all respects with material previously prepared,x can be derived by preparative TLC (30% ethyl acetate-chloroform) of the crude reaction mixture. Irradiation of methanolic solutions of the isobutenyl-2-pyridone (no precautions were taken prior to irradiation to exclude water from the methanol) led to production of the pyridonylethanol derivative. Adventitious water presumably is partaking in this reaction. Irradiation of Aqueous Solutions of 1-( l-trans-Propenyl)-2-pyridones. Preparation of 1-(2-HydroxypropyI)-2-pyridone (8).A solution of I - ( I-rmns-propenyl)-2-pyridone8 (0.20 g, 1.5 mmol) in 250 mL of degassed, deionized water was irradiated in the preparative apparatus for 3.0 h using Pqrex glass filtered light. h'itrogen was bubbled through the solution during irradiation. Concentration of the photoo 0. I9 g of a golden oil ( 8 I%)characterized by lysate in ~ a c u gave spectroscopic methods as pure I -(2-hjdroxypropyl)-2-pyridone.x U V absorption spectrophotometric monitoring of the photolysis and workup showed the following changes: maxima a t 302 (before irradiation). 287 (crude photolysate bcforc concentration), and 303 nni (after concentration). The colorless. crjstalline pyridonyl alcohol 8 (0.079 g, 35%. mp 95-96.5 "C), identical in all respects with material previously prepared.8 can be derived by preparative TLC (30°/0 ethyl acctate-chloroform) of the crude reaction mixture. irradiation of 1-(2-Methyl-l-propenyl)-2-pyridone in .4queous
3615
Anal. Calcd for C9Hi2NCIO6: C, 43.30; H, 4.84; N, 5.61; CI, 14.20. Found: C, 43.12; H, 5.09; N. 5.78; C1, 14.05. Irradiation of the isobutenyl-2-pyridone in aqueous solutions containing formic acid led to formation of the 5.5-dimethyloxazolo[3,2nlpyridinium formate salt, which was not purified directly but rather converted to the perchlorate salt by perchlorate ion exchange using DOWCX-I-X~. Irradiation of 1-( 1-trans-Propenyl)-2-pyridonein Aqueous Perchloric Acid Solution. Preparation of 5-Methyloxazolo[3,2-a]pyridinium Perchlorate (10). A solution of I -( I -rrans-propenyl)-2-pyridone8 (0.250 g. I .90 mmol) i n 450 mL of degassed, deionized water containing 0.3 mL of 70% perchloric acid ( 1 2 mmol) was irradiated in the preparative apparatus under a nitrogen atmosphere using Pyrex glass filtered light for 3 h. Concentration in vacuo in the photolysate gave a gold oil which crystallized upon addition of 5 mL of chloroform to yield 21 7 mg (50%) of pure 5-methyloxazolo[3,2-a]pyridinium perchlorate: mp 124-126 "C: ' H NMR (D20) 6 2.1 (d. 3 H , J = 7 Hz. CH?), 5.50 (AB q, 2 H, J = I I Hz, methylene), 6.00 (m,1 H. inethine H-5), 7.85 (d, 1 H. J = 1 1 Hz, pyridinium H-2), 7.93 ( 1 H, t. J = 9 Hz, pyridinium H-5), 8.80 (ni. 2 H. pyridinium H-4 and H-6); "C N M R (DzO) 6 160.2 (s, pyridinium C-2). 147.9 (d, pyridinium C - 6 ) , 140.0, 138.3. and 128.5 (d, pyridinium C-3. C-4. and C-5). 106.0 (d, C - 5 ) . 56.8 (t, C-4), 19.6 (q, CH3); U V (HzO) max 285 nm ( t 4740); IR (Nujol) 1635, 1575, 1080 cm-l; mass spectrum m/e (re1 intensity) (for parent minus HC104 fragment) 135 ( 5 0 ) , 134 (57), 120 ( 1 00), 95 (40). 67 (53); high-resolution mass spectrum m/e (for parent minus HC104) 135.069 10 (calcd for CsHgNO, 135.068 40). Attempts to obtain elemental analyses for this salt were unsuccessful, presumably owing to its extreme hygroscopic nature. Reactions of the Oxazolo[3,2-a]pyridinium Salts 9 and 10 with Potassium Hydroxide. Solutions of 5,5-dimethyl- and 5-methyloxazolo[3.2-a]pyridinium perchlorate ( I 37 mg (0.9 nimol) and 120 mg (0.8 mmol). respectively) in 4 mL of methanol containing equimolar amounts of potassium hydroxide were independently stirred at room temperature for 12 h. The reaction mixtures were concentrated in vacuo. giving oils which were purified by preparative TLC (30% ethyl acetate-chloroform). This procedure yielded the dimethyl- and methylpyridonylethanols 7 and 8 in yields of 105 (77%) and 95 mg (79%),respectively. Spectroscopic data on these compounds matched that reported pre;iously.8 Reactions of the Oxazolo[3.2-alovridinium Salts 9 and 10 with Potassium terf-Butoxide. Soiutions"& 5,5-dimethyl- and 5-methyloxazolo[3.2-a]pyridinium perchlorate (90 mg (0.4 mmol) and 100 mg (0.4 mmol), respectively) in tert-butyl alcohol (25 mL) and potassium tert-butoxide (from l .Og ofpotassium) were independently heated a t 50 "C under argon for 4 h . These solutions, after addition of water. were concentrated in vacuo. giving oils which were subjected to preparative TLC (chloroform) giving I -(2-methyl-I -propenyl)?-pyridonc (82 nig. 91%) and a mixture ofcY.c- and trans-I-(I-propcnyl)-l-pyridone(87 mg. 8790). respectivelq. Spectroscopic data for thcsc compounds were identical nith those previously reported.x Pyrolyses o f the Oxazolo[3,2-a]pyridinium Salts 9 and 10. Cryst:illinc dimethyl- and mcthyloxazolopyridinium perchlorates (50 mg (0.10 mmol) and 60 mg (0.20 nimol). reapcctivel>\ were heated independently a t 150 "C under argon for I h . The oils obtained were Perchloric Acid Solution. Preparation of 5.5-Dimethyloxazolol3,2purified b j preparative TLC (chloroform), qielding 37 mg (74%) of B lpyridiniurn Perchlorate(9). A solution of I -(?-methyl- I-propen41)I-(2-nieth~l-2-propcnql)-2-pyridone and 5 I nig (85%) of l-(Z-pro2-p)ridone8 (0.250 g, 1.7 mmol) in 450 mL of degassed. deioni7ed pcnj I)-2-pyridone. respectively. These substances displayed spec\+;iter containing 0.3 mL of 70% perchloric acid ( I Z mmol) M ~ I Si r r i l troscopic properties equivalent to those obtained for independently diatcd in the preparative apparatus under a nitrogcn atmosphere using prepared niateria1s.x Pjrcx glnss filtered light for I .5 h . The disappearance of the isobuThis pyrolytic reaction can be conducted in the injector block (220 tcnqlpjridone and appearance of the pqridiniuni salt Mere monitored "C) during GLC analysis. Accordingly, GLC analyses of aqueous by U V spectroscopic changes in the 250-320-nm region. The photosolutions of the oxazolopyridinium salts showed peaks with retention lysate was concentrated in vacuo. giving an oil which crystallized upon times identical with those of the corresponding allyl- and methallyladdition of 5 mL o f chloroform yielding 188 mg (45%) of pure 5,spyridones. dirncthyloxazolo[3.2-n]pyridinium perchlorate: mp 156- I58 "C: IH Dark Control Experiments. A variety of dark control reactions were NMR(D~O)h2.20(s,6H.CH-(),5.25(s.ZH,methylene),7.85(d, attempted using I -(2-methyl-I-propenyl)-2-pyridone and related I H. J = I I .O Hz. pyridinium H-2). 7.93 ( I H, t. J = 9.0 Hz.pyrisubstances. ( I ) A solution of this pyridone ( I 00 mg) and p-toluenediniuni H-5). 8.80 (m. 2 H. pyridinium H-4 and H-6): I3C NIMR sulfonic acid ( I I5 mg) in I 5 mL of water was heated at 75 "C under (CDC13) h 159.3 (s, pyridinium C-2). 147.9 (d, pqridinium C-6), argon for I 2 h . The crude reaction mixture was extracted with chlo140.8. 140.2. and 139.5 (d, pyridiniuni C-3, C-4, and C-5). 92.0 (5, roform. The chloroform extracts were dried and concentrated in vacuo, C-5). 61.2 (I. C-4). 26.6 ( 4 . CH?): L'V (H20) max 285 nm (6 5140); giving quantitative recovery o f the pure starting isobutenylpyridone. IR (Nujol) 1650, 1575, and 1085 cm-I; mass spectrum m/e (re1 i n (2) The isobutenql-2-pyridone was heated at 150 "C under argon for tensity) (for parent minus HC104 fragment) 149 (37). 148 (42), 134 I h . Unreacted starting material has recovered quantitatively. (3) A ( 1 OO), I28 (68). and I27 (41); high-resolution mass spectrum m/e (for solution of I -(2-hydroxy-2-methylpropyl)-2-pyridone(100 mg) and parent minus HCIOJ fragment) 149.084 60 (calcd for C 9 H I I N 0 , 90% formic acid (0.1 mL) in 10 mL of methanol was heated at reflux 149.084 06). for I 2 h . Concentration in vacuo of the crude reaction mixture gave
3616
Journal ofthe American Chemical Society
quantitative recovery of the starting pyridonylethanol derivative. Irradiation of 1-(2-Methyl-2-propenyli-2-pyridone (11). A solution of I -(2-methyl-2-propenyI)-2-pyridonex(250 mg, 1.68 mmol) in 450 mL of degassed, deionized water, containing 0.3 mL of 70% perchloric acid, was irradiated in the preparative apparatus under nitrogen using Pyrex glass filtered light. Disappearance of the starting pyridone was monitored by U V spectroscopic methods and found to be nearly complete after 5.0 h. Concentration in vacuo of the crude photolysate gave a mixture which was shown not to contain any of the 5.5-dimethyloxazolo[ 3,2-a]pyridinium perchlorate salt or corresponding pyridonylethanol derivative 7. Further characterization of the complex reaction mixture was not attempted. Irradiation of 1-Vinyl-2-pyridone (13). A solution of 1 -vinyl-2pyridonel" (250 mg, 2.07 mmol) in 450 mL of degassed, deionized water was irradiated under nitrogen in the preparative apparatus using Pyrex glass filtered light for 18.5 h. Concentration in vacuo of the crude photolysate gave a mixture which was shown by spectroscopic and chromatographic methods to contain a number of materials of high molecular weight none of which corresponded to the pyridonylethanol derivative. Triplet-Sensitized Photochemical Studies of 1-(2-MethyI-l-propenyll-2-pyridone ( I ) . The following solutions were prepared: I, 1 -
(2-methyl- l-propenyl)-2-pyridone (50 mg, 3.36 mmol) and 90% formic acid (100 pL) in 100 mL of methanol containing I .3 X IOF5 M xanthone; II,the isobutenylpyridone (50 mg) in 100 mL of methanol containing 1.3 X M xanthone; Ill,the isobutenylpyridone (50 mg) and 100 p L of 90% formic acid in 100 mL of methanol containing I .3 X M Michler's ketone; IV, the isobutenylpyridone (50 mg) in 100 mL of methanol containing 1.3 X M Michler's ketone; V, the isobutenylpyridone (50 mg) in 100 m L o f methanol; V I , the isobutenylpyridone (50 mg) and 100 p L of 90?h formic acid in 100 mL of methanol; V I I , 100 mL of methanol containing 1.3 X M xanthone; VI11 100 p L of 90% formic acid in 100 mL of methanol containing 1.3 X M xanthone; IX, 100 m L of methanol containing I .3 X M Michler's ketone; X, I00 p L of 90% formic acid in 100 mL of methanol; V I I , 100 mL of methanol containing 1.3 X M xanthone; V I I I , 100 p L of 90% formic acid i n 100 mL of M Michler's ketone. These solutions methanol containing I .3 X were degassed and placed individually into sealed Pyrex glass tubes. Simultaneous irradiation of these tubes was performed using uranium glass filtered light for 3 h. Analyses of the crude photolysates were performed using U V spectroscopic methods and C L C ( 5 ft X '/x in., I .4% OVlOl on Anasorb, I50 "C, 20 mL/min). Solutions I-IV containing the light-absorbing triplet sensitizers were shown to contain unreacted sensitizer and isobutenylpyridone. Solutions V and VI were shown to contain trace amounts of the pyridonylethanol derivative and oxazolopyridinium salt. respectively. Solutions VI I and V l l l were shown to contain the photoreduction products of xanthone.Ilb Preparation of I-(2-Methyl-l-propen?;Ii- and I-(1-trans-Propenyll-2-hydroxypyridinium Perchlorate (16 and 171. Solutions of 1 (2-methyl-I -propenyI)-2-pyridonex ( 1 00 mg. 0.67 mmol) and of 1 -
( I -trans-propenyl)-2-pyridone8 (250 mg. I .85 mmol) in 10 niL of 0.54 M perchloric acid in methanol were individually stirred under nitrogen
at room temperature for I2 h. The reaction mixtures were coticentrated in vacuo, giving products as waxy solids characterized as the desired hydroxypyridinium salts 16 (98 mg, 98%. m p 75-77 "C) and 17 (240 mg, 97'Yo, mp 68-71 "C). Spectroscopic properties of these compounds are as follows. 16: H %MR (CDCI,) 6 2.2 and 2.5 (d, 6 H, CH,). 7.1 (br s, 1 H, vinyl), 7.7 (overlapping t and d. 2 H, pyridinium H-3 and H - 5 ) . 8.7 (overlapping t and d. 2 H pyridinium H-4 and H-6); LV (0.5 M HC104/CH30H) max 285 n m ( e 5220). 17: IH NMR (CDC13) 6 2.5 (d. 3 H. CH,), 6.8 (dq, 1 H,@-vinyl),7.7 (m, 3 H, a - v i n y l and pyridinium H-3 and H-5), 8.7 (overlapping d and t, 3 H, pyridinium H-44 and H-6): L V (0.5 M HCIOJ/CH,OH) max 1 8 1 nin (c 7820). Irradiation of I-(2-Methyl-l-propenyl)- and 1-( 1-trans-Propenyl)2-hydroxypyridinium Perchlorate (16 and 17). Solutions of I -(2mcthyl- I -propenyl)- and 1 - ( I -trans-propenyl)-2-hydroxypyridinium
perchlorate (50 mg) in 100 m L of 0.54 M. degassed. methanolic perchloric acid within Pyrex glass tubes uere independently, siniultancously irradiated using unfiltered light. GLC analysis ( 5 ft X I/# in.. 1.4% OVlOl on Anasorb, 150 O C , 20 m L / m i n ) of the crude photolysates after 3 h showed nearly total decomposition of the atarting perchlorate salts and no conversion to the corresponding oxa7olo[3,2-a]pyridinium salts. Irradiation of Tetrahydrofuran Solutions of 1-(2-Methyl- I-propenyll-2-pyridone (1) of Varying Water Concentration. Solutions con-
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101:13
/ June 20, 1979
taining 50 mg (0.34 mmol) of I -(2-methyl-I -propenyl)-2-pyridone in 100 mL of solvent consisting of the following composition of tetrahydrofuran and water (percent by volume), 100% THF, 75% THF-HrO, 50%THF-H20,25%THF-H20. and 100% HrO, were degassed and irradiated simultaneously using Pyrex glass filtered light for I h. GLC analysis (5 ft X '/s in., I .5% OVlOl on Anasorb. I50 OC, 20 mL/min) using I-vinyl-2-pyridone as an internal standard gave the results recorded in Figure 2. Quantum Yield Measurements. Quantum yields were measured using a "linear optical bench" system equipped with a high-pressure, 500-W mercury lamp (Osram HBO-500-Wi2). the output ofwhich was focused with a quartz collimator and passed through a quartzfaced. watcr-cooled. three 1 -cm compartment filter solution cell containing, separately. 2 M NiSOq6HzO in 5% HrS04, saturated CoS04-7H20 in 5% H2S04, and 7.2 X IO-4 M K2Cr04 i n 0.2% NaOAc. This solution filter combination gave relative transmission characteristics before and after irradiations as follows: 230 nm, 109/0; 250 nm, 0%; 300 nm, 35%; 31 5 nm, 79%; 325 nm, 60%: 335 nm. 7%. The filtered light then passed through two quartz-faced, water-cooled cells aligned in series. During actinometry runs, both the front and back cells were filled with 0.006 M potassium f e r r i ~ x a l a t e .During '~ photolysis runs, the front cell contained 1 -(2-methyl-l-propenyl)2-pyridone solutions and the back cell potassium ferrioxalate in order to monitor light not absorbed by the substrate. Light output was determined before and after each photolysis and was found to be exceptionally constant. Product analyses for reactions conducted i n aqueous acetonitrile solutions were performed by C L C (5 ft X I/$ in., 1.5% OVlOl on Anasorb, I20 "C, 20 mL/min) using I-vinyl-2-pyridone as an internal standard. Product analyses for reactions conducted in aqueous acid wlution involved initial treatment of the crude photolysate with stoichiometric amounts of potassium hydroxide to convert the oxarolopyridinium salt to pyridonylethanol derivative followed bq GLC. Conversions in quantum yield runs were maintained in the region of 8.- 18%. Quantum yields for product formation obtained under a variety of conditions are recorded in Table II. The quantum yields for conversion of the isobutenylpyridone to the corresponding Dewar pyridone were measured using 5.4 X I O w 3 M solutions of pyridone in freshly distilled tetrahydrofuran. acetonitrile. and methanol under N2 at 25 "C. Product analyses were performed bq CLC (5 ft X l/g in., 1.5% OVlOl on Anasorb. 120 "C. 20 mL/min) using benzophenone as an internal standard added after irradiation. The quantum yields were 0.261 at 5.9% conversion in tetrahydrofuran, 0. I43 at 3.1% conversion in acetonitrile, and 0.06 I at 1.4%conversion in methanol. Fluorescence Quantum Yields. Fluorescence measurements were taken using a Spex Fluorolog. The observed emission was shown to correspond to fluorescence of the substrates by the identities of excitation and absorption curves and by the mirror image relationships betibeen excitation and emission curves. The relative fluorescence q u a n t u m yiclds of the ,\'-vinylpyridones and related substances were mcnsured in degassed solutions at 25 "C according to the method of Parker and Rees" using benzene ( $ J f = 0.07 in ~ y c l o h e x a n eand ~~) naphthalene ($1 = 0.23 in c y ~ l o h e x a n eas ~ ~standards. ) Singlet Lifetime Measurements. Singlet lifetimes were measured using a single photon counting. nanosecond spectrofluorometer based upon ORTEC components. Solutions containing the ,V-vinylpyridones and related substances were degasscd prior t o and maintained at 25 "C during nicasureiiients. Valucs for T were calculated by a Digital RXOI computer using the method of nionients program. The inhtrument M ~ calibrated S using the quinine bisulfate standardization met Irradiation of I-(2-Methyl-l-propenyl)-2-pyridone ( I ) in Deuterium Oxide. A solution of I -(2-niethyl- I -propenyl)-2-pyridone (50 iiig, 0.34 nimol) in 50 mL of degassed deuterium oxide was irradiated in a Pyrex tubc for 6 h. Concentration of the photolysate in vacuo gave a gold oil which itas purified by preparative TLC (chloroform) yielding 18 mg of I - ( 2-methyl-2-hydroxypropyl)-2-pyridone. H N MR analysis of this material (integration of proton resonances at 6 4.1 (a-CH2) and 7.4 (ring (u-CH) showed that one deuterium atom had been incorporated at the a-methylene position of the alcohol side chain. Mass hpcctrometric analqsis confirmed that only one deuterium had been incorporated: parent, mie 168.100 40 (calcd for CsHizDNOz, 168.100 90) and P-I (0%).
'
2,5-Dimethyl-2,4-hexadiene Quenching of 1-(2-MethyI-l-propenyll-2-pyridone (1) Fluorescence and Reaction. Quantum Yields and Singlet Lifetimes. Singlet lifetimes and fluorescence quantum yields
Mariano, Leone / Excited-State Chemistry of I -Alkenyl-2-pyridones were measured, using the techniques described above, with degassed solutions at 25 "C containing 6.0 X M 1-(2-methyl-l-propenyl)-2-pyridone in 50% MeOH-CH3CN (v/v) and varying concentrations of 2,5-dimethyl-2,4-hexadiene. Using these conditions all light is absorbed by the isobutenyl-2-pyridone. The results of this experiment are given i n Figure 1 , Relative quantum yields for production of 1 -(2-methyl-2-hydroxypropyl)-2-pyridone were determined using simultaneous irradiations of degassed solutions (50% CH30H-CH3CN (v/v), containing 1(',-methyl- l-propenyl)-2-pyridone ( 2 5 nig, 0.17 mmol, in 25 mL of solvent) and varying concentrations of 2,5-dimethyl-2,4-hexadiene, with Pyrex glass filtered light (all light absorbed by the pyridone), for 1.5 h. The photolysates were analyzed by GLC (5 ft X in., '/g in., I .5% OVlOl on Anasorb, 120 "C, 22 mL/min) using I-vinyl-2-pyridone as an internal standard. The results of this experiment are displayed in Figure 1 .
3617
tolysis of xanthone in methanol have been described.'lC These were used in this work. (c) V. Zanker and E. Ehrhardt, Bull. Chem. SOC.Jpn., 39, 1694 (1966). [a) L. M.Stephenson, D.G. Whitten, G. F. Vesley. and G. S.Harnmond, J. Am. Chem. SOC., 88,3665 (1966); L. M. Stephensonand G. S.Hammond Angew. Chem., Int. Ed. Engl., 8, 261 (1969). (b) Indeed, quenching of I s 1 by tetramethylbutadiene would have led to the observation of reaction quenching even if the alcohol 3 arises via the triplet of I.However, we would have fully expected much more efficient quenching of IT> by this should possess a greater diene at the hi h concentrations used, since ITl lifetime than 1 7 and the triplet quenching rate constant should be k,,,. Also, if reactions were occurring from both singlet and triplet excited states, then a biphasic quenching curve would have been obtained. C. G. Hatchard and C. A. Parker, Proc. R. Soc. London, Ser. A, 235, 518 (1956). P. S. Mariano, E. Krochmal, and A. Leone, J. Org. Chem., 42, 1122 (1977). The preparation and photochemistry of these Nvinylamides will be described at a future daie. (16) J. Hoch. C. R. Acad. Sci., 200, 938 (1935). (17) P. J. Kropp, 2. L. F. Gaibel, E. J. Reardon, K. E. Willard, and J. H. Hattaway, J. Am. Chem. SOC.,95, 7058 (1973). and references cited therein. Acknowledgments. Financial support of this research ini(18) G. Frater and H. Schmid. Helv. Chim. Acta. 50. 255 (1967). i19) W. M. Horspool and P. L: Pauson, Chem. Conhun., j 9 5 (i967). tially by the National Institutes of Health (CA- 16695) and (20) A. Albert. "Heterocyclic Chemistry," Athlone Press, London, 1968, p currently by the Robert A. Welch Foundation, the Department 80. of Energy (EC-77G-055621), and the Texas A & M University (21) J. W. Bridges, D. S. Davies, and R . T. Williams, Biochem. J., 98, 451 (1966). Center for Energy and Mineral Resources is acknowledged. (22) (a) A. G. Schultz and M. B. DeTar. J. Am. Chem. Soc., 96, 296 (1974);(b) W e would like to acknowledge the exceptionally competent S. H. Groen, R . M. Kellog, J. Butler, and H. Wynberg, J. Org. Chem., 33, 2218 (1968). and generoils assistance given to us by Professor Janos Fendler (23) (a) W. A. Henderson and A. Zweig. Tetrahedron Left., 625 (1969); (b) J. A. and Michael Hall in developing methods for photophysical Elix, D. P. H. Murphy, and M. V . Sargent. Synth. Commun., 2,427 (1972); measurements and statistical analyses. In addition, Dr. Emil A. G. Schultz and R . D. Lucci, J. Org. Chem., 40, 1371 (1975). (24) (a) Ylides of this nature have been prepared previously by reactions of Krochmal is thanked for his technical assistance with initial munchnones with reactive aldehydes and ketones.24b(b) E. Funke, R. photochemical studies. Huisgen, and F. C. Schaefer, Chem. Ber., 104, 352 (1971); R. Huisgen. E. Funke, H. Gotthardt, and H. L. Panke, ibid., 104, 1532 (1971). (25) R. K. Howe and K. W. Ratts, Tetrahedron Lett., 4743 (1967). (26) (a) Although the rate constants for protonationof delocalized carbon bases References and Notes are often smaller than that for bimolecular diffusion,26bthe assumption (1) Preliminary reports of portions of these studies have appeared: P. S. k = 1 X 1O'O M-' s-l in this case appears valid since this value (9.95 Mariano. A. A. Leone, and E. Krochmal, Tetrahedron Lett, 2227 (1977); 10') gives the smallest errors for k,, kpo, and k,,, by the nonlinear P. S. Mariano and A. A. Leone, J. Am. Chem. Soc., 100, 3947 (1978). least-squaresanalytical methods used. In addition, the value for km obtained (2) Camille and Henry Dreyfus Teacher-Scholar grantee, 1975-1980. using this method and assumption appears reasonable.26bPerhaps the (3) (a) E. C. Taylor and P. 0. Kahn, J. Am. Chem. Soc., 85, 776 (1963); (b) L. degree of charge delocalization to the ylide 28 is low, thus accounting for A. Paquette and G. Slomp, ibid., 85, 765 (1963); (c) E. J. Corey and J. proton transfer rate constants characteristic of localized anionic bases. Streith, ibid., 86,459 (1964); (d) R. C. Deselmsand W. R. Scheigh. Tetra(b) M. Eigen, Angew. Chem., Int. Ed. Engl., 3, 1 (1964). (27) i. Zugravescu and M. Petrovand, "Nitrogen Ylid Chemistry." McGraw-Hili. hedron Lett., 3563 (1972); (e) H. Furrer, Chem. Ber., 105, 2780 (1972). (4) (a) N. C. Yang and G. R. Lenz, Tetrahedron Lett., 4897 (1967); (b) R. W. New York, 1976, p 204, and references cited therein. Hoffman and K. R. Eicken, ibid., 1759 (1968). (28) (a) H. Shizuka, T. Takayama. T. Morita, S.Matsumoto, and i. Tanaka, J. Am. (5) (a) The analogous oxadi-a-methane rearrangement has been well exploChem. SOC.,93, 5987 (1971); (b) E. W. Forster, K. H. Grellman, and H. red5b.5cand one example of an oxadi-a-amine rearrangement has been Linschitz, ibid., 95, 3108 (1973). presented;5d(b) H. E. Zimmerman, S. S.Hixson, and P. S. Mariano, Chem. (29) L. J. Sharp and G. S. Harnmond, Mol. Photochem., 2, 225 (1970). (30) E. Grunwald and S. Winstein, J. Am. Chem. Soc., 78, 2770 (1956); S.W. Rev., 73, 531 (1973); (c) K. N. Houk, ibid., 76, l(1976); (d) A. Padwa and E. Vega, J. Org. Chem., 40, 175 (1975). Winstein, E. Grunwald, and H. W. Jones, ibid., 73, 2700 (1951). (6) No examples of the di-a-amine rearrangement to our knowledge have been (31) Based upon a predicted Yvalue for THF in the range of ca. -6 using data found but the familiar di-a-methane rearrangement has been well explorecorded in C. Reichardt, Angew. Chem., Int. Ed. Engl., 4, 29 (1965). red.5b (32) Preliminary studies have shown that water serves to effectively quench (7) (a) Chapman-type photocy~lizations~~ are prevented in 1-alkenyl-2-pyridone the familiar 1,3-acyi shift reactions4a of Nphenyl-Nisobutenylacetasystems by structural constraints; (b) 0. L. Chapman, G. L. Elian. A. Bloom, mide. and J. Ciardy, J. Am. Chem. Soc., 93, 2918 (1971). (33) This process might be responsible for the conversion of cu-acylamino(8) P. S.Mariano, E. Krochmal, P. L. Huesmann, R. L. Beamer, and D. Duna&thioalkylacrylamides to amidoyloxazoles: C . J. Veal and D. W. Young, way-Mariano, Tetrahedron, in press. Tetrahedron Lett., 2985 (1976). (9) (a) The spectroscopic properties of Nmethyl Dewar pyridone have been (34) A heteroatom analogue of this cyclization process appears to be operating described by C ~ r e and y ~ those ~ of the Ndeuterio material by D e ~ e l m s . ~ ~ in the cyclization reactions of 10-phenyiisoalloxazinesoccurring upon ir(b) Spectroscopic properties of the [ 4 t 41 dimer derived from N radiation in acid solution: W. R. Knappe. Chem. Ber., 107, 1614 (1974). methyl-2-pyridone have been detailed by Taylor3aand P a q ~ e t t e . ~ ~ (35) C. A. Parker and W. T. Rees. Analyst, 85, 587 (1969). (10) H. Kaye and S. H. Chang, Tetrahedron, 26, 1369 (1970). (36) I. E. Berlman. "Handbook of Fluorescence Data of Aromatic Molecules," (11) (a) S. L. Murov, "Handbook of Photochemistry." Marcel Dekker, New York, 2nd ed., Academic Press, New York, 1971. 1973. (b) Accurate methods for analysis of pinacol formation from pho(37) W. H. Melhuish. J. Phys. Chem., 65, 229(1961).
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